1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and more particularly to an electrophotographic photoreceptor in which lowering of electric resistance thereof is prevented.
2. Description of the Prior Art
There have been known a number of types of electrophotographic photoreceptors of different structure and a number of electrophotographic processes have been known in the art. As typical electrophotographic photoreceptors, a photoreceptor comprising a substrate and a photoconductive layer formed thereon, and a photoreceptor comprising a substrate, a photoconductive layer and an insulating layer laminated in this order have been known and employed widely in image formation. The photoreceptor comprising a substrate and a photoconductive layer is employed in the most typical electrophotographic process comprising charging, exposure to imagewise light and development, and further transfer of the developed image when required. In the photoreceptor having an insulating layer, the insulating layer is provided for the purpose of protecting the photoconductive layer, improving the mechanical strength of the photoreceptor and conforming the photoreceptor to a specific electrophotographic process. Representative examples of the photoreceptors having an insulating layer and the processes conducted therewith are described in, for example, U.S. Pat. No. 2,860,048 and Japanese Patent Publication Nos. 16,429/1966, 15,446/1963, 3,713/1971, 23,910/1967, 24,748/1968, 19,747/1967 and 4,121/1961.
An electrophotographic photoreceptor is required not only to have characteristics necessary for the electrophotographic process such as photosensitivity and electric and optical characteristics, but also to exhibit stability of the characteristics when used under various conditions. Nevertheless, an electrophotographic photoreceptor employing a photoconductor which releases free ions is deteriorated by moisture. That is, in such an electrophotographic photoreceptor, it is not possible to obtain a high electric potential in the unexposed area of the electrostatic latent image and a high contrast potential thereof when the electrostatic latent image is formed in the atmosphere of high humidity.
Since such a photoconductor is generally dispersed in a binder to form a photoconductive layer, various improvements in the binder have heretofore been attempted to prevent the deterioration by moisture. For example, Japanese Patent Publication No. 33,861/1975 discloses the use of watersoluble copolymer of itaconic acid monoalkyl ester, itaconic acid dialkyl ester and specific vinyl monomer as a binder of photoconductive zinc oxide to obtain an electrophotographic recording sheet exhibiting less deterioration when used in an atmosphere of high humidity. Also, Japanese Patent Application Laid Open No. 93,149/1975 discloses the formation of moistureproof layer comprising a deionization-treated material such as polyvinyl alcohol, gelatin and gum arabic over the surface of a photoconductive layer provided on a substrate to obtain an electrophotographic photoreceptor having improved resistance to moisture. Further, Japanese Patent Publication No. 13,582/1976 discloses the use of insulating resinous binder containing a composition comprising alkylated silica and a blending resin such as phenolic resin, polyamide resin and polyester resin to obtain an electrophotographic photoreceptor having improved resistance to moisture. However, although the above-mentioned publications disclose the prevention of deterioration by moisture of the electrophotographic photoreceptor by improving the resistance to moisture thereof, they do not deal with or suggest the prevention of deterioration by moisture of the photoconductor itself. Further, when the resistance to moisture of the photoreceptor is elevated by improving the binder employed in the photoconductive layer thereof as described above, unevenness is generally apt to arise in the resulting image.
Japanese Patent Publication No. 33,740/1976 discloses an electrophotographic photoreceptor using a photoconductor which is deionization-treated by means of ion exchange resin to prevent the deterioration by moisture. However, the deionization treatment disclosed in this publication is conducted to remove residual ions contained in the photoconductor before the photoconductor is incorporated into the photoreceptor. Therefore, this publication does not show the removal of free ions which are generated in the photoreceptor when the photoreceptor is used repeatedly.